BEIRUT -- U.S. Secretary of State John Kerry called Russia's plan to withdraw its forces from Syria "a very important phase in this process" and said he would go to Moscow next week to talk with Russian President Vladimir Putin and Foreign Minister Sergey Lavrov.
On Tuesday, Putin ordered most of the estimated 3,000 to 6,000 personnel to begin withdrawing from Syria.
"As we mark the fifth anniversary of the start of this horrific war, we may face the best opportunity that we've had in years to end it," Kerry said at the White House on Tuesday alongside the Georgian foreign minister.
Kerry acknowledged the serious challenges that remain, including potential violations of the cease-fire. But he sounded a note of cautious optimism.
"It's also clear that the violence in Syria is down, very significantly reduced," he said. "And access to humanitarian assistance is up, though it still could be increased even further. The cessation of hostilities is now going into its third week, defying most predictions, and far more importantly, improving the lives of Syrians on a daily basis."
Russia's plan to withdraw forces is sending a strong message to President Bashar Assad, whose hard-line stance is diverging from Moscow's interest in declaring its intervention in the country a success while also accelerating peace efforts.
Having dramatically turned the tide of war in Assad's favor with five months of intense bombardment of his foes, President Vladimir Putin is pressuring the Syrian leader to engage them in more meaningful dialogue in talks that have begun in Geneva.
"There was an overlap in interests in the last few months. Now they (the Russians) are telling Assad, 'this is where we start to diverge, and you've got to step up to your responsibilities, you can't rely on us forever,'" said Maha Yahya, acting director of the Carnegie Middle East Center.
At a televised meeting Monday with his foreign and defense ministers, Putin said Moscow's intervention had fulfilled its objectives by allowing Assad's military to "radically" turn the tide of war. He added that the move should help serve as a stimulus for Syria's political talks.
Russia deployed its air force to Syria in September to prop up Assad's faltering military, which has been waging a five-year war against internal opponents and jihadist militants.
Although its stated goal was to fight Islamic State of Iraq and Syria (ISIS) and other terrorists, much of the Russian campaign has targeted mainstream rebels and helped eject them from core areas considered strategic for Assad's survival, thereby safeguarding Moscow's interests in the country.
While the operation has restored momentum for Assad's forces, Syrian forces have been unable to regain areas in Idlib province in the north or completely encircle rebels in the contested city of Aleppo, for instance.
The timing of the Russian withdrawal, just as peace talks were resuming, offered Putin an opportune moment to declare the bulk of Moscow's involvement to be over, while acting as a peacemaker and helping ease tensions with NATO member Turkey and the Gulf monarchies vexed by the Kremlin's military action.
By also pacifying the opposition, Putin has set up the groundwork for what is shaping up to be the best opportunity so far to advance the talks between the two warring sides.
The U.N.'s Syria envoy, Staffan de Mistura, said it is a "significant development, which we hope will have a positive impact on the progress of the negotiations in Geneva." Syrian opposition spokesman, Salem Al Mislet, also welcomed Russia's pullout, saying that if it is serious, it would go a long way in helping the talks.
While Russia is highly unlikely to drop Assad anytime soon, the withdrawal at least suggests differences between Moscow and Damascus over what the next steps forward should be.
As Assad's forces regained momentum, Assad has taken a more hard-line position, saying he will continue fighting until he recaptures every inch of Syrian territory.
Syrian Foreign Minister Walid al-Moallem held a news conference over the weekend in which he said any talk of removing Assad during a transitional period sought by the U.N. is "a red line," and he rejected the international call for a presidential election to be held within 18 months -- a key opposition demand.
Assad also has called for parliamentary elections to be held as scheduled next month in government-held areas of the country.
"For Assad, this is a very long-term fight. I don't think it's a fight that Putin necessarily wants his country to be part of. This is not his Vietnam," Yahya said.
On Tuesday, Putin's spokesman Dmitry Peskov denied that Russia's decision was prompted by the Kremlin's displeasure with the Syrian government's tough position in the negotiations, or that it was intended to put pressure on Assad.
The withdrawal announcement also triggered successive statements from the Syrian presidency and armed forces rejecting speculation that the decision reflected a rift between the allies.
Militarily, the implications of Russia's drawdown are unlikely to hurt Assad as long as Iranian-organized Shiite militias are still fighting on his behalf. Russia, which is keeping its naval base in Tartous and air base in Latakia, may also redeploy at any time if needed.
Hossein Royvaran, a political analyst in Tehran, said the withdrawal is part of a plan agreed upon by Iran, Syria and Russia, and that Moscow's forces may return to Syria if the political process fails.
But Firas Abi Ali, senior analyst at IHS Country Risk, said the withdrawal highlights divergences among Iran, Syria, and Russia, and probably reflects Moscow's intention to impose a compromise that might include a partition or federation model.
"Russia can accept a settlement in which the Kurds gain autonomy in northeastern Syria, Sunni groups dominate Idlib and Aleppo, while a successor to the Assad government remains in core Syrian government territory in Damascus, Homs, and along the coast," Abi Ali said. "This could take the form of a new federal constitution, or even Syria's partition."
On Monday, Lavrov said it would be up to the Syrians to decide what form of state they should have.
Several analysts, however, dismiss talk of any formal partition or federal system based on sectarian and ethnic identities that would be sowing the seeds for further conflict down the road.
Mark Galeotti, a New York University global affairs professor who spends much of his time in Moscow, said the Russians have been signaling that they think it's time for some kind of resolution in Syria.
"It means that Russia will either have to trim (Assad's) sails a bit or that it might be that it's time for him to go," he said.